(x+5)(15-3x)=0

Solution for (x+5)(15-3x)=0 equation:

(x+5)(15-3x)=0

We add all the numbers together, and all the variables

(x+5)(-3x+15)=0

We multiply parentheses ..

(-3x^2+15x-15x+75)=0

We get rid of parentheses

-3x^2+15x-15x+75=0

We add all the numbers together, and all the variables

-3x^2+75=0

a = -3; b = 0; c = +75;
Δ = b2-4ac
Δ = 02-4·(-3)·75
Δ = 900
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{900}=30$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-30}{2*-3}=\frac{-30}{-6}
=+5
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+30}{2*-3}=\frac{30}{-6}
=-5
$